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Desilication of Caledonian granites in the Barnesmore complex, Co. Donegal: The origin and significance of metasomatic syenite bodies
Author(s) -
Dempsey C. S.,
Meighan I. G.,
Fallick A. E.
Publication year - 1990
Publication title -
geological journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.721
H-Index - 54
eISSN - 1099-1034
pISSN - 0072-1050
DOI - 10.1002/gj.3350250319
Subject(s) - geology , geochemistry , feldspar , metasomatism , quartz , biotite , mineral , mineralization (soil science) , alkali feldspar , petrogenesis , mineralogy , mantle (geology) , paleontology , chemistry , organic chemistry , soil science , soil water
The ca. 400 Ma Caledonian Barnesmore complex, Donegal, includes a 1·5 km 2 area in the Main Granite (G2) in which pristine biotite monzogranite has undergone in situ alteration to brick‐red, two feldspar, chloritic syenite which is virtually quartz ‐free. The principal chemical changes involve major elements (Si, Al, Ca, Na, K) rather than trace elements. Stable isotope mineral data (O, H) show essentially magmatic values. It is proposed that a silica‐under‐saturated, volatile (halogen?) — and alkali (+Al)‐enriched, late‐stage, aqueous magmatic fluid was channelled into vertical conduits, dissolving quartz, precipitating feldspar(s) and disrupting the fabric of the host granite. The resulting syenitic bodies and vents exposed elsewhere in the complex are believed to represent different levels in such pipes. The mineralogical and chemical modification recorded here has important implications for the study of granitic rocks elsewhere in the world in that more subtle degrees of alteration may have escaped recognition and other equally drastic examples may have associated mineralization potential.